WO 94/01203 discloses a system with a furnace that supplies flue gas to a first fluidized bed and then to a boiler. From the boiler the flue gas is supplied to a carbonator. The first fluidized bed includes solid particles that are heated by the flue gas and circulate between the first fluidized bed and a fluidized bed calciner, for heating the fluidized bed calciner. At the fluidized bed calciner a sorbent used in a fluidized bed carbonator is regenerated. At the fluidized bed carbonator CO2 from flue gas generated in the furnace is adsorbed.
In this system at the fluidized bed calciner a large amount of CO2 is required for fluidization; this CO2 must be first cooled before passing a fan and then reheated, these steps are associated with costs for the equipment and increased energy consumption. In addition the transfer of sorbent to the furnace would be associated with a sever loss of specific sorbent surface resulting in sorbent deactivation.
WO 2012/152899 discloses a system for the calcination of raw material for cement production; the system has a fluidized bed in which a fuel is combusted in presence of solid particles. From the fluidized bed the solid particles are separated from the flue gas and are sent to a calciner. Raw meal and CO2 are supplied at the bottom of the calciner and rise through the calciner, and the solid particles are supplied at the top of the calciner and fall by gravity through the calciner in counter flow to the rising raw meal and CO2; the solid particles transfer heat and calcine the raw meal.
Also, this system is conceived for the calcination of fresh limestone (having a higher CO2 content than recarbonated sorbent), application of this system to spent sorbent (having a reduced CO2 content) would require additional fluidization gas (CO2) for compensation resulting in increasing equipment and operating costs. In addition, the specific pressure drop over such a counter current solids flow bed is characteristically high and the partial pressure of CO2 (which is typically high at the bottom of this bed) strongly increases the required temperature to effect calcination. As such, the calcination of the rising particles is late and the cooling of the larger solid particles impeded; the sorbent temperature at the bottom of the bed increases forcing an increase in the exit temperature of the solids. The required heat flux for calcination combined with the limited cooling of the solid particles at the bottom of the bed implies the circulation of an unnecessarily large amount of solids particles.
WO 2009/105419 discloses a system for reducing carbon dioxide emissions from gas generated in burning fossil fuel.
The system has a combustion vessel in which a fuel is combusted. The vessel contains inert solid particles that are heated during the combustion and sorbent particles that adsorb carbon dioxide generated during the combustion. The solid particles are transferred by falling through the combustion vessel to a calcination device and the sorbent particles rich in carbon dioxide are also supplied to the calcination device by entrainment from the combustion device and subsequent separation and transport. In the calcination device, the inert solid particles and the sorbent particles rich in carbon dioxide behave like a moving bed, causing heat transfer from the solid particles to the sorbent particles, causing carbon dioxide release from the sorbent particles. Carbon dioxide is then discharged from the calcination device and the inert solid particles and sorbent particles are allowed to fall into a second moving bed area where they are cooled for recirculation to the combustion vessel.
Also in this system, at the calcination device where inert solid particles and sorbent particles exchange heat in a moving bed and where calcination takes place and CO2 is produced, an increase in the local calcination temperature can be expected due to back pressure caused by CO2 bubble formation and the movement of the released CO2 through the bulk, acting to reduce the temperature driving force at the bottom of the bed.
Also inefficiencies associated with mal distribution of inert sorbent particles and sorbent particles entering the calcination device lead to inefficiencies in heat transfer inside the moving bed, ultimately impeding heat transfer from the inert solid particles, limiting the cooling and the final temperature achieved of the inert solids and further the specific fraction of heat which can be extracted from a given mass of circulating inert solid particles.
Also sorbent particle entrainment in falling clusters of larger inert solids which fall to the lower section of the combustion device would intensify sorbent deactivation at the increased local temperatures and require compensation through additional make-up material or an increased solids circulation rate.